On Sun, Dec 4, 2016 at 6:45 PM, David Menendez
Surely you can do the same with the constructor class.
newtype Ab a = Ab (a -> Bool) instance Siftable Ab where siftAway _ = Ab (const False) sift p (Ab g) = Ab ...
Ab is contravariant, so you would need something like
siftContraMap :: (a -> Maybe b) -> f b -> f a
That's the Functor version, which entirely excludes contravariant things. For the plain sift :: (a -> Bool) -> f a -> f a version, you can use instance Siftable Ab where sift f (Ab g) = Ab (\x -> g x && f x) siftAway (Ab g) = Ab (const False)
I'm not sure if my siftAway excludes anything it shouldn't....
I’m not sure it’s possible to define siftAway so that it isn’t equal to sift (const Nothing).
As you pointed out, your laws for the plain Siftable don't exclude sift _ = id for an arbitrary Siftable. Adding siftAway with that law ensures that sift (const Nothing) actually "empties" the container. I doubt it actually makes sense to add it to the API, though. As for names, I think for consistency with the rest of the world, the method names that make the most sense are filter, mapMaybe, filterM, and traverseMaybe. The filterM name is a bit unfortunate, since it only needs an Applicative constraint, but that seems to be what people like. The monad-extras package uses the name mapMaybeM, but that strikes me as a terrible name because it's really much more like traverse than like map. David